Belt tensioning means for use in photocopying are know, for example, U.S. Pat. Nos. 4,183,658 and 4,592,641. See also U.S. Pat. No. 4,114,536. In the photocopier disclosed in U.S. Pat. No. 4,183,658, a photoconductive belt on which an image is electrophotographically formed is pressed against an intermediate support in order to transfer the formed image to the support. At the point where the contact pressure is applied, the belt assumes the speed of the intermediate support so that the image transfer can take place without smearing.
In this copier, the intermediate support is driven at a speed slightly less than that of the photoconductive belt so that during the image transfer the loop formed in the belt by a first floating roller in front of the point of contact pressure becomes larger and the loop formed by a second floating roller after contact, becomes smaller. If the contact pressure is eliminated after the image transfer, the second floating roller, which exerts a tensioning force greater than that of the first floating roller, enlarges the loop it forms in the belt after the point of contact and at the same time reduces the loop that the first floating roller forms in the belt in front of the contact point. Thus, after each image transfer the original position of the photoconductive belt with respect to the intermediate support is restored while the belt remains tensioned.
In order to avoid disturbing impacts and vibrations in the belt during the movement of the floating rollers from one position to the other, the difference between the forces exerted by the two floating rollers is preferably controlled to be no larger than the force required to move the rollers and that part of the belt therebetween. Also, the force which each roller exerts separately is preferably made no larger than necessary to keep the associated belt loop sufficiently taut.
One method of controlling such forces has been to use the weight of the floating rollers and/or tension forces produced by springs acting on the floating rollers. However, in practice, difficulties often occur in the operation of such devices due to the cumulative effect of deviations from various components due to accepted manufacturing tolerances which result in imprecise control of the weight on balancing. Accordingly, it is an object of the invention to obviate such difficulties by providing a novel tensioning means.